Dresden 2003 – wissenschaftliches Programm
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O: Oberflächenphysik
O 38: Struktur und Dynamik reiner Oberfl
ächen
O 38.8: Vortrag
Freitag, 28. März 2003, 13:00–13:15, FOE/ANOR
Short-range order at CoAl(100) surfaces — •S. Müller, O. Wieckhorst, L. Hammer, and K. Heinz — Universität Erlangen-Nürnberg, Lehrstuhl für Festkörperphysik, Staudtstr.7, 91058 Erlangen
As we have recently shown [1], the surface composition of B2-ordered Co1+єAl1−є alloys is controlled by Co anti-site atoms which segregate to the top layer of the compound. Although these anti-sites experimentally clearly show tendencies to short-range ordering, up to now in theory the top layer was modelled either as random alloy or long-range ordered structure. We show that short-range ordering can be treated by the combination of first-principles calculations, cluster expansions, and Monte-Carlo simulations. For 30% Co-antisites in the top layer and T = 200K, we find the formation of Co chains along the [011] direction. These Co chains are mostly separated by at least two diagonal Al-rows characteristic for the (3× 3)diag structure. For higher antisite concentrations (xCo = 0.50) chains of Co atoms along [001] and [010] directions are found, in agreement with the presented ground-state diagram. Increasing the temperature from T = 200K to T = 500K destroys a large precentage of the characteristic Co chains along [011], however, the ordering still shows clear deviations from an ideal random alloy (supported by DFG).
[1] V. Blum et al., Phys. Rev. Lett., in press